New paper provides no evidence that polar bears ate whale carcasses to survive Eemian interglacial

Contrary to what the misleading press release implies, an entirely speculative new paper by polar bear specialists Kristin Laidre and Ian Stirling (among others) presents zero evidence that polar bear consumed whale carcasses during the last warm Interglacial (Eemian, ca. 115-130kya). And contrary to the impression that Eemian conditions were very challenging for polar bears, simulations from the single paleo sea ice simulation paper these authors cite show the ice-free season over most of the Eemian was less severe than today in the polar basin, with no reason for polar bears to scavenge extensively on large whale carcasses.

LaidreFEE_Wrangel Island scavenging_smaller

Polar bears are shown scavenging on the carcass of a dead bowhead whale that washed ashore on Wrangel Island, Russia. Credit: Chris Collins/Heritage Expeditions

This is yet another paper posing as science co-authored by Stirling that uses anecdotal accounts of behaviour to send a message about evolutionary capabilities of polar bears (Stirling and van Meurs 2015). With little or nothing to back it up, the paper’s real purpose is to convey Stirling’s opinion that past polar bear survival is irrelevant to understanding future polar bear survival — and that all the bears are gonna die unless we do something about carbon dioxide emissions generated by fossil fuel use.

Is it a coincidence that the Summary for Policy Makers was issued by the IPCC over the weekend (not the report with the science in it but the document that all politicians agreed were acceptable)? Look no further than the last sentence of National Geographic’s article on this story, which includes a quote from lead author Laidre and a link to the magazine’s interpretation of the new IPCC report:

“Laidre put it even more bluntly: “If you want polar bears around we need sea ice, and loss of sea ice closely tied to our activities and our fossil fuel emissions.” (Learn about the IPCC’s dire new climate report.)” 

The only reference to what sea ice might have been like during past warm Interglacial periods is this statement from the Laidre et al. paper (my bold):

“Thus, polar bears survived through the warmest interglacial period (the Eemian, 130–115 kya) and possibly three or more interglacial periods prior to the Eemian. Although some sea ice remained in all seasons (Stein et al. 2017), total cover and duration were greatly reduced and remained primarily over the deep and relatively unproductive polar basin…”

In fact, Stein et al. used models based on “low resolution” proxies from a few locations in the Arctic Basin to simulate past ice cover. They concluded that sea ice in March was virtually identical to pre-industrial times (i.e. more than the present) and only slightly less abundant in June.

Only in late summer (i.e. the seasonal minimum in September) did model results suggest that significantly less ice could have been present during the Eemian (the Last Interglacial or LIG) than in pre-industrial times, when CO2 levels were the same (ca. 290 ppm). However, this is more ice during September than we’ve seen for the last decade or so, even though temperatures in NE Siberia during the Eemian were about 9 degrees Celsius higher than today (all of which contradicts the notion that CO2 is the primary driver of Arctic sea ice coverage).

In other words, there is no evidence provided from the literature that polar bears survived through an ice-free season that was longer than present (or longer than projected for the future) and no reason for a reduced population of Arctic seals to have existed at that time in late fall through early spring (when bears do most of their feeding). Therefore, there is no reason for polar bears to have needed to utilize large whale carcasses any more during the summer than they might do today.

The concluding sentence from the Laidre et al. paper states:

“If warming continues unmitigated, temperatures in the Arctic will exceed those documented for the past million years by the middle of the century, making it almost impossible to use history to predict the future.”

Conditions of the future referred to in the above statement were based on a single sea ice modeling paper that used predicted carbon dioxide emissions from fossil fuels as the primary driver (even though similar models have been very inaccurate). But compare it to a statement from a similar paper by Stirling and van Meurs 2015:

“…increased diving ability cannot evolve rapidly enough to compensate for the increasing difficulty of hunting seals because of the rapidly declining availability of sea ice during the open-water period resulting from climate warming.”

From the press release

Polar bears gorged on whale carcasses to survive past warm periods, but strategy won’t suffice as climate warms

Michelle Ma, UW News

Polar bears likely survived past warm periods in the Arctic, when sea ice cover was low, by scavenging on the carcasses of stranded large whales. This food source sustained the bears when they were largely restricted to land, unable to roam the ice in search of seals to hunt.

A new study led by the University of Washington found that although dead whales are still valuable sources of fat and protein for some polar bears, this resource will likely not be enough to sustain most bear populations in the future when the Arctic becomes ice-free in summers, which is likely to occur by 2040 due to climate change. The results were published online Oct. 9 in the journal Frontiers in Ecology and the Environment.

“If the rate of sea ice loss and warming continues unmitigated, what is going to happen to polar bear habitat will exceed anything documented over the last million years. The extremely rapid pace of this change makes it almost impossible for us to use history to predict the future,” said lead author Kristin Laidre, a marine biologist at the UW’s Polar Science Center and associate professor in the School of Aquatic and Fishery Sciences.

Polar bears need sea ice to survive because it is an essential platform for hunting seals, their main food source. They travel over the ice, searching for breathing holes or seal birth dens. When the ice breaks up in late spring, polar bears in some populations will fast on land, waiting for the ice to re-form so they can resume hunting.

Still, polar bears are opportunistic feeders and have been observed in multiple locations eating the carcasses of whales that died at sea and washed ashore. The bears can quickly consume and store large amounts of fat, which works in their favor. In some cases, between 40 and 60 different polar bears have been observed feeding on large bowhead and gray whale carcasses and, in 2017, more than 180 bears were seen scavenging on a single dead bowhead whale. Individual bears frequently return to the same carcass over multiple years.

The authors drew upon years of observations in the field to assess the potential importance of whale carcasses and how they might help polar bears survive an ice-free Arctic. It is clear that polar bears persisted through low-ice interglacial periods in the past that resulted from naturally occurring climate cycles. The researchers hypothesized that, to a significant degree, the bears likely survived by scavenging on whale carcasses, storing large amounts of fats when hunting seals was not an option.

“I think this is likely one of the most probable explanations for how polar bears made it through previous warm interglacial periods,” said co-author Ian Stirling, former research scientist with the Canadian Department of Environment and an adjunct professor at the University of Alberta, who has studied polar bears for 45 years.”

For more information, contact Laidre at klaidre@uw.edu and Stirling at 780-993-5380 or ian.stirling@ualberta.ca.

Read it all here. Notice that a citation for the paper is not provided, or even it’s title.

Canadian Press interpretation (Bob Weber, 9 Oct 2018): “Ancient polar bears survived low ice periods on dead whales, study shows.” Like the headline of the press release, it suggests the new paper provided evidence that this behaviour indeed occurred. That could not be further from the truth, which is that the paper is a self-serving piece of speculation about what might have or could have happened in the past in order to state it definitely could never happen in the future. In fact, this is yet another paper that is more advocacy than science.

References

Laidre, K.L., Stirling, I., Estes, J.A., Kochnev, A. and Roberts,J. 2018. Historical and potential future importance of large whales as food for polar bears. Frontiers in Ecology and the Environment doi:10.1002/fee.1963

Stein, R., Fahl, K., Gierz, P., Niessen, F. and Lohmann, G. 2017. Arctic Ocean sea ice cover during the penultimate glacial and the last interglacial. Nature Communications 8 (373). https://www.nature.com/articles/s41467-017-00552-1

Stirling, I. and Ross, J.E. 2011. Observations of cannibalism by polar bears (Ursus maritimus) on summer and autumn sea ice at Svalbard, Norway. Arctic 64:478-482. http://arctic.journalhosting.ucalgary.ca/arctic/index.php/arctic/article/view/4147 Open access. Pdf here.

Stirling, I. and van Meurs, R. 2015 in press. Longest recorded underwater dive by a polar bear. Polar Biology http://link.springer.com/article/10.1007/s00300-015-1684-1

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